Macross Frontier

[KrayZ33]

It doesn't make sense for Zero to have developed the variable fighters for any purpose but that.
.

Well I'm just quoting Wikipedia here but it says

"In the original series, the reason behind the development of the variable fighters' Battroid mode was the discovery of the existence of the Zentradi race, giant humanoid aliens genetically engineered for military conquest. Although at the time the humans hadn't met any actual Zentradi, the discovery of the SDF-1 Macross and its accommodations for humanoid giants justified the development of a weapon that could enable mankind to fight the aliens on their own environment (Battroid mode), on the ground (Gerwalk mode), and the air (Fighter mode). Thus, research into Overtechnology culminated in the development of the variable fighters for the U.N. Spacy."

well, there is nothing else to say about that.
I think the plane Shin pilots was only produced 40 or 50 times, so it isn't much more than a prototype of the VF... they are probably the first to test in combat.

And at least the U.N. forces had a reason to develop the VF-0
because of the SV-51 variable fighter of the anti-u.n. forces.

those machines brought in a new kind of aerial combat (clearly superior to the f-14 shin used at the beginning before he was shot down) and thus there was the need for a fighting plane similiar to those.